Basically yes, 4s is the cap so if you don't have enough skill in it, you will get between 2-4s on the stun.

The kinetic damage should increase even after you invest enough skill to hit the duration cap, with more skill invested. However in general, I would say the damage is not worth investing for, but if you want it you can continue to invest in the relevant skills and get more damage.

Just an addition: Even without any points into Photonic Theory, you can easily obtain a 3.6 second stun with points in Emitters and Deflectors (using either a Borg or Aegis Deflector, even).